It is well known that when metallic ion emitters are subjected to corona discharges in room air, they show signs of deterioration within a few hours and the generation of fine particles. This is known for needle tips, copper, stainless steel, aluminum, and titanium emitters. Corrosion is found in areas under the discharge or subjected to the active gaseous species NO.sub.X. NO.sub.3 ions are found on all the above materials, whether the emitters had positive or negative polarity. Also, ozone-related corrosion is dependent on relative humidity and on the condensation nuclei density.
Silicon and silicon dioxide emitter electrodes experience significantly lower corrosion than metals in the presence of corona discharges. It is also known that by purging the emitter electrodes with dry air, NH.sub.4 NO.sub.3 can be reduced as either an airborne contaminant or deposit on the emitters.
Surface reactions lead to the formation of compounds that change the mechanical structure of the emitters. At the same time, those reactions lead to the generation of particles from the electrodes or contribute to the formation of particles in the gas phase.
Silicon is known to undergo thermal oxidation, plasma oxidation, oxidation by ion bombardment and implantation, and similar forms of nitridation.
Prior art includes U.S. Pat. Nos. 5,447,763; 5,047,892; 5,057,966; 4,967,608; 3,789,278; 3,813,549; 4,110,614; 4,837,658; 5,539,205; 5,596,478; 5,116,583; and Japanese patent 7-70348. None of these, nor of many technical papers in the field teach or suggest the present invention.